Eddy Current

Image distortion due to field gradient eddy currents can create image artifacts in diffusion-weighted MR images. These images, acquired by measuring the attenuation of NMR signal due to directionally dependent diffusion, have recently been shown to be useful in the diagnosis and assessment of acute stroke and in mapping of tissue structure. This work presents an improvement on the spin-echo (SE) diffusion sequence that displays less distortion and consequently improves image quality. Adding a second refocusing pulse provides better image quality with less distortion at no cost in scanning efficiency or effectiveness, and allows more flexible diffusion gradient timing. By adjusting the timing of the diffusion gradients, eddy currents with a single exponential decay constant can be nulled, and eddy currents with similar decay constants can be greatly reduced. This new sequence is demonstrated in phantom measurements and in diffusion anisotropy images of normal human brain.

Abstracf-Various magnetic vector potential formulations for the eddy current problem are reviewed. The uniqueness of the vector potential is paid special attention. The aim is to develop a numerically stable finite element scheme which performs well at low and high frequencies, does not require an unduly high number of degrees of freedom, and is capable of treating the case of multiply connected conductors.

Aliased and dealiased numerical simulations of a turbulent channel flow are performed using spectral and finite difference methods. The problem of aliasing errors in spectral simulations Analytical and numerical studies show that aliasing errors are more of turbulent flows has been discussed in the literature [4-7]. destructive for spectral and high-order finite-difference calculations It has been demonstrated that, depending on the form of than for low-order finite-difference simulations. Numerical errors the nonlinear terms [5] in the Navier-Stokes equations, have different effects for different forms of the nonlinear terms in aliased spectral simulations can become unstable, exhibit the Navier-Stokes equations. For divergence and convective forms, spectral methods are energy-conserving only if dealiasing is per-decay, or give reasonable results. Zang [5] performed sevformed. For skew-symmetric and rotational forms, both spectral eral spectral simulations of transition and turbulence in and finite-difference methods are energy-conserving even in the incompressible flow and reported that, without dealiasing, presence of aliasing errors. It is shown that discrepancies between the simulations with convective and divergence forms of the results of dealiased spectral and standard nondialiased finitethe nonlinear terms were numerically unstable, whereas difference methods are due to both aliasing and truncation errors with the latter being the leading source of differences. The relative the computations with the rotational form produced inacimportance of aliasing and truncation errors as compared to subgrid curate results. The aliasing errors associated with the rotascale model terms in large eddy simulations is analyzed and distional form were reported to be even more damaging in cussed. For low-order finite-difference simulations, truncation ersimulations of transition. The poor behavior of the rotarors can exceed the magnitude of the subgrid scale term. ᮊ 1997 tional form was also reported earlier by Horiuti [8] who Academic Press carried out large eddy simulations of turbulent channel flow. No dealiasing was performed in that study. Horiuti found that turbulence decayed when the rotational form 310

Patient motion and image distortion induced by eddy currents cause artifacts in maps of diffusion parameters computed from diffusion-weighted (DW) images. A novel and comprehensive approach to correct for spatial misalignment of DW imaging (DWI) volumes acquired with different strengths and orientations of the diffusion sensitizing gradients is presented. This approach uses a mutual information-based registration technique and a spatial transformation model containing parameters that correct for eddy current-induced image distortion and rigid body motion in three dimensions. All parameters are optimized simultaneously for an accurate and fast solution to the registration problem. The images can also be registered to a normalized template with a single interpolation step without additional computational cost. Following registration, the signal amplitude of each DWI volume is corrected to account for size variations of the object produced by the distortion correction, and the b-matrices are properly recalculated to account for any rotation applied during registration. Both qualitative and quantitative results show that this approach produces a significant improvement of diffusion tensor imaging (DTI) data acquired in the human brain.

The proper orthogonal decomposition technique (Lumley's decomposition) is applied to the turbulent flow in a channel, to extract coherent structures by decomposing the velocity field into characteristic eddies with random coefficients. In the homogeneous spatial directions a generalization of the shot-noise expansion is used to determine the characteristic eddies. In this expansion the Fourier coefficients of the characteristic eddy cannot be obtained from second-order statistics. Three different techniques are used to determine the phases of these coefficients: (i) a technique based on the bispectrum, (ii) a spatial compactness requirement, and (iii) a functional continuity argument. Results from these three techniques are found to be very similar. The implications of these techniques and the shot-noise expansion are discussed in the Appendix. The dominant eddy is found to contribute as much as 76% to the turbulent kinetic energy. In two and three dimensions, the characteristic...

The conceptional design of the proposed linear electron-positron collider TESLA is based on 9-cell 1.3 GHz superconducting niobium cavities with an accelerating gradient of E acc ≥ 25 MV/m at a quality factor Q 0 ≥ 5 • 10 9. The design goal for the cavities of the TESLA Test Facility (TTF) linac was set to the more moderate value of E acc ≥ 15 MV/m. In a first series of 27 industrially produced TTF cavities the average gradient at Q 0 = 5 • 10 9 was measured to be 20.1 ± 6.2 MV/m, excluding a few cavities suffering from serious fabrication or material defects. In the second production of 24 TTF cavities additional quality control measures were introduced, in particular an eddy-current scan to eliminate niobium sheets with foreign material inclusions and stringent prescriptions for carrying out the electron-beam welds. The average gradient of these cavities at Q 0 = 5•10 9 amounts to 25.0±3.2 MV/m with the exception of one cavity suffering from a weld defect. Hence only a moderate improvement in production and preparation techniques will be needed to meet the ambitious TESLA goal with an adequate safety margin. In this paper we present a detailed description of the design, fabrication and preparation of the TESLA Test Facility cavities and their associated components and report on cavity performance in test cryostats and with electron beam in the TTF linac. The ongoing R&D towards higher gradients is briefly addressed.

Previous analytical work has shown that it is possible to design surface PM (SPM) machines using fractionalslot concentrated windings to achieve wide speed ranges of constant power operation by satisfying the optimal fluxweakening condition. This paper presents a 6kW 36-slot/30-pole concentrated winding prototype SPM machine that has been designed using a closed-form analytical technique developed specifically for this class of machines. Experimental testing of this machine has been carried out to determine its performance capabilities, including flux-weakening operation.

A method is presented to correct for the imperfections of spatial In the following we introduce an alternative method to encoding gradients in MRI. The approach is simple and fast, can measure actual k-space trajectories in MRI. This method be performed with standard scanner hardware, and does not require does not involve the use of a reference phantom, is easy to separate measurements with reference phantoms. The new method, implement, and requires only two pulse sequence repetitions using the MR signal to accurately measure the k-space trajectory of per spatial encoding axis. the imaging sequence, allows for correction of gradient hardware imperfections and eddy-current effects. Initial measurements are METHODS presented which demonstrate the efficacy of the method to correct images acquired with spiral and EPI scan techniques. The new method measures the actual k-space trajectories

The electromagnetic wave-absorption properties of Nd 3 Fe 68Àx Mn x Co 18 B 11 (x ¼ 0, 1, 2) alloys obtained by rapid quenching from the melt was studied. The complex permittivity-frequency and permeability-frequency properties were determined in the microwave frequency regime of 2-18 GHz by vector network analysis. XRD spectra showed that only a-Fe diffraction peak was observed in the asspun alloys. It is found that the acquired complex permittivity and permeability values match the microwave frequency when the 1 at% Mn content was doped. A minimum reflection loss of À6.9 dB is obtained at 2.7 GHz for composite Nd 3 Fe 66 Mn 2 Co 18 B 11 with absorber thickness of 1.5 mm. The exchange interaction was attributed to the microwave absorption properties. The results suggest a new design of microwave absorbers based on electromagnetic wave-absorbing materials. r

A complete, three phase transformer model for the calculation of electromagnetic transients is presented. The model consists of a set of state equations solved with the trapezoidal rule of integration in order to obtain an equivalent Norton circuit at the transformer terminals. Thus the transformer model can be. easily interfaced with an electromagnetic transients program. Its main features are: (a) the basic elements for the winding model are the turns, (b) the complete model includes the losses due to eddy currents in the windings and in the iron core, (c) the solution of the state equations is obtained in decoupled iterations. For validation, the frequency response of the model is compared with tests on several transformers. Applications to the calculation of transients are given for illustration.

Finding an optimal solution for QoS-aware Web service composition with various restrictions on qualities is a multi-objective optimisation problem. A popular multi-objective genetic algorithm, NSGA-II, is studied in order to provide a set of optimal solutions for QoS-based service composition. Experiments with different numbers of abstract and concrete services confirm the expected behaviour of the algorithm.

This review of design principles for implementation of a spiral inductor in a silicon integrated circuit fabrication process summarizes prior art in this field. In addition, a fast and physics-based inductor model is exploited to put the results contributed by many different groups in various technologies and achieved over the past eight years into perspective. Inductors are compared not only by their maximum quality factors ( max ), but also by taking the frequency at max , the inductance value ( ), the self-resonance frequency ( SR ), and the coil area into account.

This paper introduces the first comprehensive and accurate compact resistance-inductance-capacitanceconductance (RLCG) model for through-silicon vias (TSVs) in 3-D ICs valid from low-to high-frequency regimes, with consideration of the MOS effect in silicon, the alternating-current (ac) conduction in silicon, the skin effect in TSV metal, and the eddy currents in the silicon substrate. The model is verified against electrostatic measurements as well as a commercial full-wave electromagnetic simulation tool and subsequently employed for various performance (delay) analyses. The compact model is also applicable to TSVs made of carbon nanotube (CNT) bundles, once a slight modification (making the effective conductivity complex) is made. Various geometries (as per the International Technology Roadmap for Semiconductors) and prospective materials (Cu, W, and single-walled/multiwalled CNTs) are evaluated, and a comparative performance analysis is presented. It is shown that CNT-bundle-based TSVs can offer smaller or comparable high-frequency resistance than those of other materials due to the reduced skin effect in CNT bundle structures. On the other hand, the performance (delay) analysis indicates that the performance differences among different TSV materials are rather small. However, it is shown that CNTs provide an improved heat dissipation path due to their much higher thermal conductivity. In addition, the improved mechanical robustness and thermal stability of CNTs also favor their selection as TSV materials in emerging 3-D ICs.

We have studied ways of predicting power losses in soft magnetic laminations for generic time dependence of the periodic magnetic polarization (). We found that, whatever the frequency and the induction waveform, the loss behavior can be quantitatively assessed within the theoretical framework of the statistical loss model. The prediction requires a limited set of preemptive experimental data, depending on whether or not the arbitrary () waveform is endowed with local slope inversions (i.e., minor hysteresis loops) in its periodic time behavior. In the absence of minor loops, such data reduce, for any peak polarization value p , to the loss figures obtained under sinusoidal () at two different frequency values. In the presence of minor loops of semiamplitude m , the two-frequency loss experiment should be carried out for both peak polarization values p and m. Additional knowledge of the quasi-static major loop, to be used for modeling hysteresis loss, does improve the accuracy of the prediction method. A more general approach to loss in soft magnetic laminations is obtained in this way, the only limitation apparently being the onset of skin effect at high frequencies. Index Terms-Eddy currents, magnetic losses, Preisach models of magnetic hysteresis, soft magnetic materials. I. INTRODUCTION S OFT magnetic materials are classified, sold, and applied on the basis of the properties determined by means of standard test methods. AC magnetic testing made according to the standards invariably requires sinusoidal induction, quite a natural working condition in magnetic cores. On the one hand, such are the typically rated voltages in electrical machines and devices and, on the other hand, there is an obvious basic aspect attached to sinusoidal induction waveforms, as building blocks of any possible periodic function. It frequently happens, however, that practical magnetic cores or substantial portions of them are subjected to nonsinusoidal flux. In some cases one can talk of distortion, that is unwanted departure from the ideal sinusoidal condition due to causes like strong nonlinearity of the material at high inductions, anisotropy (T-joints of transformers), and pulsating waveforms (teeth of stator cores). In other instances, like in variable speed motors supplied by means of pulsewidth modulated (PWM) voltages, the driving circuit itself generates non-Manuscript

We consider H(curl;)-elliptic problems that have been discretized by means of N ed elec's edge elements on tetrahedral meshes. Such problems occur in the numerical compuation of eddy currents. From the defect equation we derive localized expressions that can be used as a posteriori error estimators to control adaptive re nement. Under certain assumptions on material parameters and computational domains, we derive local lower bounds and a global upper bound for the total error measured in the energy norm. The fundamental tool in the numerical analysis is a Helmholtz-type decomposition of the error into an irrotational part and a weakly solenoidal part.

A model of core losses, in which the hysteresis coefficients are variable with the frequency and induction (flux density) and the eddy-current and excess loss coefficients are variable only with the induction, is proposed. A procedure for identifying the model coefficients from multifrequency Epstein tests is described, and examples are provided for three typical grades of non-grain-oriented laminated steel suitable for electric motor manufacturing. Over a wide range of frequencies between 20-400 Hz and inductions from 0.05 to 2 T, the new model yielded much lower errors for the specific core losses than conventional models. The applicability of the model for electric machine analysis is also discussed, and examples from an interior permanent-magnet and an induction motor are included.

This paper analyzes rotor eddy-current loss in permanent-magnet brushless ac machines. It is shown that analytical or finite-element techniques published in literature for predicting rotor eddy-current loss using space harmonic based approaches may not yield correct results in each magnet segment when one magnet-pole is circumferentially segmented into more than two pieces. It is also shown that the eddy-current loss in each equally segmented piece may differ by a large margin, which implies that the temperature distribution in the magnets will be uneven and the risk of demagnetization has to be carefully assessed. The theoretical derivation is validated by time-stepped transient finite-element analysis. Index Terms-Eddy-current loss, permanent-magnet brushless machines. Kais Atallah received the Ingenieur d'Etat degree in electrical power engineering from Ecole Nationale Polytechnique, Algeria, and the Ph.D. degree from the University of Sheffield, Sheffield, U.K. From 1993 to 2000, he was a Research Associate in the Department of Electronic and Electrical Engineering, University of Sheffield, where he is currently a Senior Lecturer. His research interests embrace fault-tolerant permanent magnet drives for aerospace, magnetic gearing, and "pseudo" direct drive electrical machines. He is a co-founder of the University spin-off company, Magnomatics, Ltd.

Permanent-magnet (PM) motors offer potential energy savings as compared with induction motors because of the virtual elimination of rotor loss and the reduction of stator loss from operation near unity power factor. In PM machines, iron losses form a significant fraction of the total loss partly due to the nonsinusoidal flux density distribution. Design optimization therefore requires good means of predicting these iron losses. Finite-element analysis can be employed but this approach is cumbersome and costly when used in the many iterations needed in optimizing the design. This paper presents a set of improved approximate models for the prediction of iron loss. They can be used in design optimization programs and, since they are directly related to machine dimensions and material properties, they also provide quick insight into the effects of design changes. A time-stepped finite-element method is employed to evaluate the iron losses in a range of typical PM machines and the results are used to evaluate the adequacy of the models. The predictions of overall iron losses are then compared with measurements made on two PM motors.

We investigate the effect of tissue heterogeneity and anisotropy on the electric field and current density distribution induced in the brain during magnetic stimulation. Validation of the finite-element (FE) calculations in a homogeneous isotropic sphere showed that the magnitude of the total electric field can be calculated to within an error of approximately 5% in the region of interest, even in the presence of a significant surface charge contribution. We used a high conductivity inclusion within a sphere of lower conductivity to simulate a lesion due to an infarct. Its effect is to increase the electric field induced in the surrounding low conductivity region. This boost is greatest in the vicinity of interfaces that lie perpendicular to the current flow. For physiological values of the conductivity distribution, it can reach a factor of 1.6 and extend many millimeters from the interface. We also show that anisotropy can significantly alter the electric field and current density distributions. Either heterogeneity or anisotropy can introduce a radial electric field component, not present in a homogeneous isotropic conductor. Heterogeneity and anisotropy are predicted to significantly affect the distribution of the electric field induced in the brain. It is, therefore, expected that anatomically faithful FE models of individual brains which incorporate conductivity tensor data derived from diffusion tensor measurements, will provide a better understanding of the location of possible stimulation sites in the brain.

Many electromagnetic actuators suffer from high velocity impacts. One such actuator is the electromechanical valve actuator, recently receiving attention for enabling variable valve timing in internal combustion engines. Impacts experienced by the actuator are excessively loud and create unnecessary wear. This paper presents an extremum seeking controller designed to reduce the magnitude of these impacts. Based on a measure of the sound intensity at impact, the controller tunes a nonlinear feedback to achieve impact velocities of less than 0.1 m/s while maintaining transition times of less than 4.0 ms. The control strategy is implemented with an eddy current sensor, to measure the valve position, and a microphone.

Small gradient fields resulting from incompletely canceled eddy currents can cause geometric distortion in echo-planar images. Although this distortion is negligible in most echoplanar applications, the large gradient pulses used in diffusion-weighted echo-planar imaging can result in significant image distortion. In this report, it is shown that this distortion can be significantly reduced by the application of bipolar gradient waveforms. Both bipolar diffusion-sensitizing gradients and an inverted gradient preparatory pulse were examined for minimizing the eddy currents responsible for these distortions.

Electromagnetic forces are generated by the movement of a conductor through a stationary magnetic field or a time varying magnetic field through a stationary conductor and can be used to suppress the vibrations of a flexible structure. In the present study, a new electromagnetic damping mechanism is introduced. This mechanism is different from previously developed electromagnetic braking systems and eddy current dampers because the system investigated in the subsequent manuscript uses the radial magnetic flux to generate the electromagnetic damping force rather than the flux perpendicular to the magnet's face as done in other studies. One important advantage of the proposed mechanism is that it is simple and easy to apply. Additionally, a single magnet can be used to damp the transverse vibrations that are present in many structures. Furthermore, it does not require any electronic devices or external power supplies, therefore functioning as a non-contacting passive damper. A theoretical model of the system is derived using electromagnetic theory enabling us to estimate the electromagnetic damping force induced on the structure. The proposed eddy current damper was constructed and experiments were performed to verify the precision of the theoretical model. It is found that the proposed eddy current damping mechanism could increase the damping ratio by up to 150 times and provide sufficient damping force to quickly suppress the beam's vibration.

The aim of this paper is to analyze a finite element method to solve the low-frequency harmonic Maxwell equations in a bounded domain containing conductors and dielectrics. This system of partial differential equations is a model for the so-called eddy currents problem. After writing this problem in terms of the magnetic field, it is discretized by Nédélec edge finite elements on a tetrahedral mesh. Error estimates are easily obtained if the curl-free condition is imposed on the elements in the dielectric domain.

This paper explains a new family of techniques to extract data from semiconductor memory, without using the read-out circuitry provided for the purpose. What these techniques have in common is the use of semiinvasive probing methods to induce measurable changes in the analogue characteristics of the memory cells of interest. The basic idea is that when a memory cell, or read-out amplifier, is scanned appropriately with a laser, the resulting increase in leakage current depends on its state; the same happens when we induce an eddy current in a cell. These perturbations can be carried out at a level that does not modify the stored value, but still enables it to be read out. Our techniques build on a number of recent advances in semi-invasive attack techniques [1], low temperature data remanence [2, 3], electromagnetic analysis [4] and eddy current induction . They can be used against a wide range of memory structures, from registers through RAM to FLASH. We have demonstrated their practicality by reading out DES keys stored in RAM without using the normal read-out circuits. This suggests that vendors of products such as smartcards and secure microcontrollers should review their memory encryption, access control and other storage security issues with care.

Nickel-zinc ferrites of different compositions, NiZnFeO with x " 0.2, 0.35, 0.5 and 0.6, have been prepared by a precursor method involving citrate precursors of the concerned metals and mixing them in solution state. Resistivity has been studied as a function of composition and sintering temperature. It is observed that NiZn ferrites prepared by this method have resistivity * 10 8 Q cm which is higher by atleast two orders of magnitude than that reported () 10 6 Q cm) for ferrites prepared by the conventional ceramic method. This is attributed to better purity as well as better compositional and microstructural control achievable by the citrate method. High resistivity makes these ferrites suitable particularly for high-frequency applications where eddy current losses are required to be low.

Summary from only given, as follows. During last years, several numerical formulations have been developed by us to modelize physical problems like: conducting film effects over the surface of insulators (pollution) [1], high frequency eddy-currents [2], earth field effects on the hull of a ship [3]. The physical effects being at stake are completely different, but in each of these examples, they originate from a region thin in regard with the other geometrical dimensions. An efficient numerical approach consists on using a surfacic representation with special boundary conditions expressing the solution inside the thin region. We propose in that paper a didactical approach to thin regions in electromanetics and, as an example, the boundary conditions and surfacic equation for eddy currents flowing inside a thin ferromagnetic shell. The numerical tests are done using the BEM software PH13D, but the results could easily be transposed in a FEM context. The practical applications may concern the computation of losses (shield of electrical machines or transformers) or low frequency electromagnetic perturbations (screen effects, EMC) as well as special domains like optimization of the induction heating of pans (french art culinaire).

The study of critical phenomena and universal power laws has been one of the central advances in statistical mechanics during the second half of the past century, explaining traditional thermodynamic critical points 1 , avalanche behaviour near depinning transitions 2,3 and a wide variety of other phenomena 4 . Scaling, universality and the renormalization group claim to predict all behaviour at long length and timescales asymptotically close to critical points. In most cases, the comparison between theory and experiments has been limited to the evaluation of the critical exponents of the power-law distributions predicted at criticality. An excellent area for investigating scaling phenomena is provided by systems exhibiting crackling noise, such as the Barkhausen effect in ferromagnetic materials 5 . Here we go beyond powerlaw scaling and focus on the average functional form of the noise emitted by avalanches-the average temporal avalanche shape 4 . By analysing thin permalloy films and improving the data analysis methods, our experiments become quantitatively consistent with our calculation for the multivariable scaling function in the presence of a demagnetizing field and finite field-ramp rate.

A systematic method to improve the quality ( ) factor of RF integrated inductors is presented in this paper. The proposed method is based on the layout optimization to minimize the series resistance of the inductor coil, taking into account both ohmic losses, due to conduction currents, and magnetically induced losses, due to Eddy currents. The technique is particularly useful when applied to inductors in which the fabrication process includes integration substrate removal. However, it is also applicable to inductors on low-loss substrates. The method optimizes the width of the metal strip for each turn of the inductor coil, leading to a variable strip-width layout. The optimization procedure has been successfully applied to the design of square spiral inductors in a silicon-based multichip-module technology, complemented with silicon micromachining postprocessing. The obtained experimental results corroborate the validity of the proposed method. A factor of about 17 have been obtained for a 35-nH inductor at 1.5 GHz, with values higher than 40 predicted for a 20-nH inductor working at 3.5 GHz. The latter is up to a 60% better than the best results for a single strip-width inductor working at the same frequency.

In echo-planar-based diffusion-weighted imaging (DWI) and diffusion tensor imaging (DTI), the evaluation of diffusion parameters such as apparent diffusion coefficients and anisotropy indices is affected by image distortions that arise from residual eddy currents produced by the diffusion-sensitizing gradients. Correction methods that coregister diffusion-weighted and non-diffusion-weighted images suffer from the different contrast properties inherent in these image types. Here, a postprocessing correction scheme is introduced that makes use of the inverse characteristics of distortions generated by gradients with reversed polarity. In this approach, only diffusion-weighted images with identical contrast are included for correction. That is, non-diffusion-weighted images are not needed as a reference for registration. Furthermore, the acquisition of an additional dataset with moderate diffusion-weighting as suggested by Haselgrove and Moore (Magn Reson Med 1996;36:960-964) is not required. With phantom data it is shown that the theoretically expected symmetry of distortions is preserved in the images to a very high degree, demonstrating the practicality of the new method. Results from human brain images are also presented.

This paper is concerned with a new modeling technique for the effective eddy current damper and vibration suppression of a beam using the eddy current damper. The eddy current damper consists of the permanent magnets and the conducting sheet. The relative motion between the magnets and the conducting sheet produces eddy currents thus resulting in the electromagnetic force, which turns out to be the damping force thus suppressing vibrations. The important advantage of the proposed eddy current damper is that it does not require any electronic devices and external power supplies. In the present study, the theoretical model for the eddy current damper is derived using the electromagnetic theory combined with the image method. The theoretical model enables us to estimate the damping augmented to the host structure as well as develop a design tool for the eddy current damper. It is found from the comparison with the experimental results that the theoretical model can predict the damping characteristics and the dynamic behavior of the structure. The theoretical and experimental results also showed that the vibration of the beam can be successfully suppressed by means of the eddy current damper.

We review the present state of understanding of the Barkhausen effect in soft ferromagnetic materials. Barkhausen noise (BN) is generated by the discontinuous motion of magnetic domains as they interact with impurities and defects. BN is one of the many examples of crackling noise, arising in a variety of contexts with remarkably similar features, and occurring when a system responds in a jerky manner to a smooth external forcing. Among all crackling system, we focus on BN, where a complete and consistent picture emerges thanks to an exactly solvable model of avalanche dynamics, known as the ABBM model, which ultimately describes the system in terms of a Langevin equation for the velocity of the avalanche front. Despite its simplicity, the ABBM model is able to accurately reproduce the phenomenology observed in the experiments on a large class of magnetic materials, as long as universal properties are involved. To complete the picture and to understand the long-standing discrepancy between the ABBM theory and the experiments, which otherwise agree exceptionally well, consisting of the puzzling asymmetric shape of the noise pulses, microscopic details must be taken into account, namely the effects of eddy current retardation. These effects can be incorporated in the model, and result, to a first-order approximation, in a negative effective mass associated with the wall. The progress made in understanding BN is potentially relevant for other crackling systems: on the one hand, the ABBM model turns out to be a paradigmatic model for the universal behaviour of avalanche dynamics; on the other hand, the microscopic explanation of the asymmetry in the noise pulses suggests that inertial effects may also be at the origin of pulses asymmetry observed in other crackling systems.

The National Spherical Torus Experiment (NSTX) has demonstrated the advantages of low aspect ratio geometry in accessing high toroidal and normalized plasma beta, βt ≡ 2µ0� p� /B 2 0 and βN ≡ 10 8 � βtaB0/Ip. Experiments have reached βt = 39% and βN = 7.2 through boundary and profile optimization. High βN plasmas can exceed the ideal no-wall

In this paper, the frequency-dependent resistance in Litz-wire planar windings for domestic induction heating appliances is analyzed. For these inductors, in which the size is not an essential constraint, an analytical model is developed based on the superposition of different loss effects in the wire. Eddy current losses, including conduction losses and proximity-effect losses, both internal and external, were considered and modeled. The magnetic field necessary to evaluate the external proximity losses is as well analytically calculated considering the complete winding and load properties. To verify this model and its limitations, several inductors with different wires and numbers of turns were constructed and results with both non-loaded and loaded inductors are compared with theoretical predictions.

... The most comprehensive analysis so far, which includes coupling of fields, circuits and mechanical motion, was carried out by Salon el ai. ... [5] VK Garg and J. Weiss, "Finite element solution of transient eddy-current problems in multiply-excited magnetic systems," ZEEE Trans. ...

To ascertain whether magnetic dynamos operate in rocky exoplanets more massive or hotter than the Earth, we developed a parametric model of a differentiated rocky planet and its thermal evolution. Our model reproduces the established properties of Earth's interior and magnetic field at the present time. When applied to Venus, assuming that planet lacks plate tectonics and has a dehydrated mantle with an elevated viscosity, the model shows that the dynamo shuts down or never operated. Our model predicts that at a fixed planet mass, dynamo history is sensitive to core size, but not to the initial inventory of long-lived, heat-producing radionuclides. It predicts that rocky planets larger than 2.5 Earth masses will not develop inner cores because the temperature-pressure slope of the iron solidus becomes flatter than that of the core adiabat. Instead, iron "snow" will condense near or at the top of these cores, and the net transfer of latent heat upwards will suppress convection and a dynamo. More massive planets can have anemic dynamos due to core cooling, but only if they have mobile lids (plate tectonics). The lifetime of these dynamos is shorter with increasing planet mass but longer with higher surface temperature. Massive Venus-like planets with stagnant lids and more viscous mantles will lack dynamos altogether. We identify two alternative sources of magnetic fields on rocky planets: eddy currents induced in the hot or molten upper layers of planets on very short period orbits, and dynamos in the ionic conducting layers of "ocean" planets with ∼10% mass in an upper mantle of water (ice).

We study the heat transfer between two parallel metallic semi-infinite media with a gap in the nanometer-scale range. We show that the near-field radiative heat flux saturates at distances smaller than the metal skin depth when using a local dielectric constant and investigate the origin of this effect. The effect of non-local corrections is analysed using the Lindhard-Mermin and Boltzmann-Mermin models. We find that local and non-local models yield the same heat fluxes for gaps larger than 2 nm. Finally, we explain the saturation observed in a recent experiment as a manifestation of the skin depth and show that heat is mainly dissipated by eddy currents in metallic bodies.

The possibility of obtaining lower space harmonic content for the fractional-slot, tooth-concentrated windings by multiplying the number of coils, leading to bigger number of layers in the armature slots is presented. By using multiple layer windings it is possible to reduce or even to cancel some space sub-harmonics having as final result lower eddy current losses induced by the armature reaction in the iron structure of the rotor. The general method of doubling (or trebling) the winding and relatively shifting by one (or more) slots is presented in the paper. Two examples of three phase concentrated windings are presented and analyzed as primitive windings, 12 slots/10poles and 9 slots/8 poles. In the first case the only existing space sub-harmonic is reduced firstly from 35.9% (from fundamental wave) to 9.6% and finally it is canceled by using different number of turns per coils. In the second case the two existing sub-harmonics are reduced and balanced by using the developed method.

The electromagnetic forming (EMF) process relies on a driving force that is induced by eddy current and magnetic field, both of which are generated in the workpiece by a transient current in a nearby coil. The high deformation rates achievable using this forming method enhances the formability of materials such as aluminum. Also, the dynamics of contact with the forming die can eliminate springback, an undesired effect that can be problematic in other forming techniques such as stamping. The advancement of the EMF technology is currently awaiting rigorous numerical modeling capabilities that can adequately simulate the forming process and be used to design the forming system. Such capabilities must be based on physical models that address the strong coupling between the electromagnetic, deformation and thermal response of the deforming workpiece and other system structure. In this paper, a brief exposition of the EMF method and its applications and a review of the existing models are given. In addition, a mathematical framework, which can be further developed numerically for the purpose of process simulation, is outlined. Finally, the paper discusses the challenges to advancing the EMF technology.

Eddy current induced in a metal by a coil carrying an alternating current may be perturbed by the presence of any macroscopic defects in the material, such as cracks, surface indentations, or inclusions. In eddy-current nondestructive evaluation, defects are commonly sensed by a change of the coil impedance resulting from perturbations in the electromagnetic field. This paper describes theoretical predictions

Resistivity and dielectric properties of the nickel-zinc ferrite composition, Ni 0.65 Zn 0.35 Fe 2 O 4 , have been studied near different sintering conditions to optimize the sintering schedule for a specific application. Variable sintering parameters in the study include sintering temperature in the range from 1150 to 1300°C and sintering time with 1 h, 2 h and 4 h durations at each sintering temperature. The results indicate that the decrease of resistivity is linear and less rapid upto a sintering schedule of 1250°C/2 h, and thereafter any increase either in sintering temperature or in sintering time causes a rapid decrease of resistivity due to the increased volatilization of zinc from the samples at these temperatures/times. Possible mechanisms contributing to conduction processes in ferrites are discussed in explaining the obtained results.

Nanomechanical resonators with fundamental mode resonance frequencies in the very-high frequency (VHF), ultra-high frequency (UHF) and microwave L-band ranges are fabricated from monocystalline silicon carbide (SiC) thin film material, and measured by magnetomotive transduction, combined with a balanced-bridge readout circuit. For resonators made from the same film, we measured the frequency dependence (thus geometry dependence) of the quality factor. We have seen a steady decrease of quality factor as the frequency goes up. This indicates the importance of clamping losses in this regime. To study this source of dissipation, a free-free beam SiC nanomechanical resonator has been co-fabricated on the same chip with a doubly clamped beam resonator operating at similar frequencies. Device testing has been performed to directly compare their characteristics and performance. It is observed that a significant improvement in quality factor is attained from the free-free beam design. In addition, from studies of resonators made from different chips with varying surface roughness, we found a strong correlation between surface roughness of the SiC thin film material and the quality factor of the resonators made from it. Furthermore, we experimentally studied the eddy current damping effect in the context of magnetomotive transduction. A high-aspect ratio SiC